In Institute of Electrical and Electronics Engineers (IEEE), Wireless Local Area Network (WLAN) is standardized in the IEEE 802.1 1 specifications, e.g. in IEEE Standard for Information technology—Tele-communications and information exchange between systems, Local and metropolitan area networks—Specific requirements, Part 1 1: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.
WLAN is a technology that mainly operates in the 2.4 GHz or 5 GHz band. The IEEE 802.1 1 specifications regulate the physical layer between access points and wireless terminals, MAC layer and other aspects to secure compatibility and interoperability between access points and wireless devices, often referred to as STA when discussing WLAN applications. WLAN is generally operated in unlicensed bands, and as such, communication over WLAN may be subject to interference sources from any number of known and unknown devices. WLAN is commonly used as wireless extensions to fixed broadband access, e.g. in domestic environments and hotspots like airports, train stations and restaurants and the like.
Recently, WLAN has been subject to increased interest from cellular network operators, not only as an extension to fixed broadband access. Instead, the interest is mainly focused on using the WLAN technology as an extension, or alternative to, cellular radio access network technologies. By use of WLAN technology as an extension to cellular radio access network technologies it is contemplated that an ever increasing wireless bandwidth demand may be handled. Cellular operators that currently serve mobile users with, e.g. any of the Third Generation Partnership Project (3GPP) technologies, Long Term Evolution, LTE, Universal Mobile Telecommunications System, UMTS, /Wideband Code Division Multiple Access, WCDMA, or Global System for Mobile communications, GSM, treat WLAN as a technology that may provide good support in their regular cellular networks. The term “operator-controlled WLAN” refers to a WLAN deployment that on some level is integrated with a cellular network operator's existing network and where the 3GPP radio access networks and the WLAN wireless access may even be connected to the same core network and provide the same services.
In WLAN systems, Multi User Multiple Input Multiple Output, MU-MIMO, and transmit Beamforming, TxBF, are based on Channel State Information, CSI, obtained by the access points, APs, during a training sequence, also known as sounding. The CSI comprises a channels estimate or in other words is a snap shot in time of the state of the channel. The natural variability of the wireless channel causes the CSI, and hence also the precoding matrices set up in the AP, to age. Such aging gives rise to loss of beamforming gain and interference cancellation over time. Thus, the CSI needs to be refreshed. However, excessive sounding wastes channel capacity without benefits and too few soundings degrade performance.
Hence there is a need to determine when precoding matrices are outdated and sounding is needed.